Nitrogen waste must leave dissolved in water, so its form depends on an animal's habitat. Aquatic animals like fish excrete ammonia directly, as they have plenty of water to dilute it. Terrestrial mammals convert it to urea, which is less toxic and can be stored, while birds and reptiles paste it out as uric acid to conserve precious water. Studying the local monitor lizards and comparing them to our own physiology, I've always found it elegant that our plumbing is a ledger of evolutionary constraints.
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